Still here and thriving like never before, two enduring American dance institutions dominate Bay Area stages this week. The Alvin Ailey American Dance Theater was founded by dancer-choreographer Ailey in 1958; Paul Taylor launched his company in 1954, even before he joined Martha Graham's troupe for eight years. Like all artistic institutions, these companies have not been immune to the inroads and inevitabilities of time.

The causes for a company's decline are many. Artistic directors retire, die, lose interest or terminate the organization, as did Merce Cunningham before he died in 2009. The financial picture clouds. Legal complications, like those that followed Graham's death, sometimes intrude.

And because they are dedicated to the most evanescent of the performing arts, all dance companies are in peril. None can survive without a substantial repertoire in reserve. Locally, Smuin Ballet picked up and continued with a new/old formula when its founder, Michael Smuin, died four years ago.

But there are no printed scores, no official documentation of dances. To restage them, one must often rely on dim videos, scribbled notes and the memories of erstwhile performers, who brought to the dance their own inflections, many of which over the years acquire the authority of gospel.

So why have Ailey and Taylor survived and prospered, while so many others have withered? Continuity happens because of generations of ardent believers in a dance artist's work.

No company better testifies to this thesis than the Alvin Ailey American Dance Theater. When Ailey died in 1989, his electrifying star dancer, Judith Jamison, became artistic director. After 21 years of artistic growth and financial stabilization (the current annual budget is $29 million, the endowment stands at $55 million), Jamison will step aside at the conclusion of this season.

Yet Jamison has planned for the company's future by appointing choreographer Robert Battle as her successor. "I chose him for his individuality," she said during a conference call from Ailey's handsome New York headquarters, the Joan Weill Center for Dance. "Battle will do what he wants to do, and his voice needs to be heard. But he will be surrounded by folks who really care about the longevity of the company."

To that end, Jamison has promoted Matthew Rushing, a dancing pillar of the company for 18 years, to the post of rehearsal director. The invaluable Masazumi Chaya, to whom Jamison has entrusted revival projects, remains associate artistic director. He retired from dancing in 1986 and seems blessed with a wondrous memory and a meticulous eye.

"The best way to retrieve a dance from the past," Jamison said, "is not to put too many cooks in the stew. You've got to have that one person. Chaya's work is as clear as a bell."

Berkeley will witness his contribution during the West Coast premiere of Ailey's "Three Black Kings" (1976) Tuesday through next Sunday at Zellerbach Hall. The piece came with Duke Ellington's last major score, and when Chaya went to work on it a few years ago, he found portions of the music missing. "To restore it, we got much help from Wynton Marsalis and Jazz at Lincoln Center," he said. Also on this visit, Jamison's stirring 1971 solo, "Cry," will be shared by three women.

Although she allows that Ailey always wanted a repertory company, rather than a showcase for his dances, Jamison refuses to allow memories of past performances to dominate the current revivals. As for "Cry":

"My line is: There's only one Lana Turner," she said. "It's not going to look the same as it was and it's not meant to. I could not possibly have danced it the way today's dancers do. The public insists on dancers with techniques that are through the roof; no way in the world that a dancer of my generation could get on the stage with them."

Comparisons aren't helpful either when it comes to Taylor, who retired from the stage in 1974. His eye for engaging dancers remains as unerring as ever. "It's a wonderful time to be in the company," said Taylor's executive director, John Tomlinson. "Paul has a fanatic following. There's almost a religious conviction about them."

That crowd is always demanding reprises of landmark Taylor dances that have slipped from the repertoire. "Paul approves revivals reluctantly," Tomlinson said. "He is most excited about going into the studio and making a new dance. Yet he is endlessly pragmatic in this matter."

The major revival of this visit - Wednesday through next Sunday at the Novellus Theater at Yerba Buena for the Arts - will be the legendary, hourlong "Orbs," created in 1966 to movements from Beethoven string quartets and absent since 1982. "The piece was created in a dense style that has not been seen recently. It provides a new perspective on Taylor's choreography," Tomlinson said.

The restoration process, steered by Taylor's vigilant rehearsal director (and former dancer), Bettie de Jong, epitomizes the company's manner of retrieving the past. She scrutinizes the old videos, brings in surviving cast members, sets the work on the current company and only then shows it to the choreographer, who, in turn, tidies it up.

Taylor keeps the active repertoire fresh by assigning to each work a dance captain, who, in lieu of performing in that dance, assists de Jong in rehearsal. "He or she becomes Paul's right hand and is responsible for keeping the work in shape." It's a blueprint for a future without Taylor, who last year turned 80.

"But I must tell you," Tomlinson said, "Paul's health is stunningly great. He still walks to the studio every day and every year creates two new dances." They add up to 135, at last count.

Nevertheless, Taylor has planned thoroughly for the coming decades. An artistic committee will work in close relationship with the board to determine what the company will be. It could be run by another choreographer, or by an artistic director who does not choreograph, or it could gradually reduce its activities and go out of business, while the dances are leased to other troupes.

The bets are, that in some way, the Paul Taylor Dance Company will survive its founder. "We have the most important thing," said Tomlinson, "a great artist prolifically making work. Without that foundation, there's no reason to go on." {sbox}